I use Gmail with Mail.app. As you know Gmail allows replying mails as someone else (by changing From header of the sent e-mail). If you don't know, you can register multiple mails to Gmail (your personal mail, work mail etc.) and you can send mail from them from one account.

Let's say you have 3 mails registered in GMail: g@gmail.com, a@a.a and b@b.b. Your account is g@gmail.com in this case. When you use GMail web interface, if you receive a mail redirected from a@a.a, when you press Reply, it sends your mail as a@a.a (or you can choose other accounts to mimic one of your e-mail addresses as sender).

However when I'm in Mail.app, I'm only able to send mail as g@gmail.com, any ideas to solve this?

Under 'Accounts' in 'Preferences' change the 'Email Address:' to the address you want to use. I don't believe you can switch addresses at send time. You'll have to add a separate account for each email address you want to send from.